1. Field of the Invention
The present invention relates to a method and apparatus for performing partial etching and pattern etching by an electrolytic reaction.
Specifically, the present invention relates to an improvement in the method and apparatus for removing a shorted portion and for device separation by etching a surface layer of a semiconductor device, such as a photovoltaic cell.
2. Related Background Art
Conventionally, thin-film etching has been carried out for device separation in a device having on a substrate a thin film of metal, a semiconductor, a transparent conductive film and the like, and for removing a short between thin films caused by mechanical stress at the cut surface. Examples of the above-mentioned device include various devices, for example, a photovoltaic cell, such as a solar battery, a sensor and the like, a light-emitting device (photoemitters), such as a photosensitive apparatus, an EL device and the like, and a transmissive or reflective device, such as a liquid crystal device, an electrochromic device and the like. In particular, for a solar battery fabricated through deposition onto a long metal substrate, it is necessary to divide the long substrate into small cells, and depending on the design it may be necessary to further electrically separate into small zone faces in the small cell interior. When divided up by cutting, the cut end surface becomes a shunt due to a short caused between the metal substrate and the transparent conductive film. Here, the word “shunt” encompasses not only a complete conductive state but also a state in which an insulation resistance value drops from slight conductance, irrespective of the amount. Even if it does not become a shunt, because the electrode at the cut surface is made bare, there is a possibility of a shunt forming as a result of stress during use. Thus, etching line formation is required as a preventative measure.
FIG. 12 is a schematic diagram showing a planar surface of an etched solar battery of a small cell that was cut from a long substrate. In FIG. 12, reference numeral 1201 denotes a substrate, reference numeral 1202 denotes an etching line inside the cell and reference numeral 1203 denotes a periphery etching line. As is clear from looking at FIG. 12, an etching line is a region that does not participate in generating electricity because the upper portion electrode is removed. Thus, from the viewpoint of effectively extracting power, it is preferable to form the lines as fine as possible. From the same perspective, it is also preferable to form the lines as far as possible on the cell edge.
FIG. 13A is a cross-sectional view of the line 13A-13A from FIG. 12, which explains the shunt state of a photovoltaic cell cross-section. In FIG. 13A, reference numeral 1201 denotes a substrate, reference numeral 1301 denotes a semiconductor layer, reference numeral 1302 denotes an upper electrode and reference numeral 1303 denotes a shunt portion. For a solar battery, the substrate 1201 consists of metal and is about 0.15 mm thick, a semiconductor layer 1301 is approximately several μm and an upper electrode 1302 is approximately several tens of nm. If the semiconductor layer 1301 is in a dark state, specific resistance is at least about 1×1010·cm. Since the upper electrode is 1×103·cm, if an above-described shunt portion is not present, the space between the substrate and the upper electrode is effectively in an insulating state. If cutting was carried out by shirring or a press machine, the upper electrode 1302, which consists of a thin film, deforms as a result of being subjected to stress, thereby coming into direct contact with the metal substrate 1201, whereby a low-resistance shunt portion is formed. In other cases, even if the deformation does not cause a direct contact, a low-resistance shunt portion is formed as a result of the damage received from the stress. In this case, normal characteristics can be restored by using etching to remove the upper electrode 1302 of the shunt portion. That is, an etching line should be formed on the position of the shunt portion 1303. FIG. 13B shows the situation after the shunt portion 1303 of FIG. 13A has been etched. Reference numeral 1304 illustrates the condition in which the upper electrode 1302 has been removed by etching. FIG. 14 is a cross-sectional view along the line 14-14 in FIG. 12. In FIG. 14, reference numeral 1202 denotes an etching line, and the other reference numerals which are also indicated in FIG. 13A denote the same members as those in FIG. 13A.
FIG. 19 illustrates the IV properties when a shunt is present, and also when a shunt is not present, in a solar battery. When a shunt is present, as in FIG. 13A, the shunt resistance Rsh is, for example, about 1000Ω, wherein the IV curve is sloped. However, when a shunt has been removed by an etching line, as in FIG. 13B, the shunt resistance Rsh is, for example, about 1000 KΩ, wherein the IV curve is much better, being something close to a rectangle.
Examples of a conventional electrolytic etching method and apparatus for forming an etching line as discussed above include, for example, the suitable methods and apparatuses disclosed in Japanese Patent Application Laid-Open No. 9-115877 and Japanese Patent Application Laid-Open No. 2002-227000. The methods disclosed in these publications employ an electrode, which comprises a working electrode for dissolving a subject etching object by an electrochemical reaction and an auxiliary electrode for suppressing diffusion of the etching region resulting from the working electrode, wherein an insulating member is formed on the working electrode and/or the auxiliary electrode by a laminating or coating process.
In the above-described conventional art, the working electrode has the same shape as the etching pattern and is positioned facing the object being etching in an extreme proximity thereto, whereby a highly accurate etching pattern can be achieved.
FIG. 8 is a schematic cross-sectional view illustrating one example of a conventional electrolytic etching apparatus. In FIG. 8, reference numeral 801 denotes a working electrode, reference numeral 802 denotes an insulating member, reference numeral 803 denotes an auxiliary electrode, reference numeral 804 denotes the electrodes in entirety, reference numeral 805 denotes a substrate of the subject etching object, reference numeral 806 denotes an electrolyte, reference numeral 807 denotes an electrolysis bath and reference numeral 808 denotes an electrolysis power source.
The working electrode 801 is an electrode for dissolving the subject etching object by an electrochemical reaction. The auxiliary electrode 803 is used as a guard electrode, which absorbs a part of the electric field from the working electrode 801 to suppress diffusion of the etching region.
FIGS. 11A and 11B are schematic sketch views illustrating the electric field distribution of a conventional electrolytic etching apparatus. FIG. 11A is a view showing the electrode position in a central portion, and FIG. 11B is a view showing the electrode position in an end surface portion. FIGS. 11A and 11B show the situation where the working electrode 1101 has a plus electric potential, the substrate 1105 is a ground (electric potential=0) and the auxiliary electrode 1103 has been impressed with the same electric potential as the substrate 1105. As can be understood from FIGS. 11A and 11B, the electric field 1108 can be broadly divided up into an electric field heading from the working electrode 1101 to the upper electrode 1107 and an electric field heading from the working electrode 1101 to the auxiliary electrode 1108. The electric field headed from the working electrode to the upper electrode is involved as it is in the reaction with the upper electrode layer, wherein the spreading condition of that electric field has a close relationship with etching line width. The electric field heading from the working electrode to the auxiliary electrode is restricted in its electric field width due to its absorption of a part of the electric field originally heading from the working electrode to the upper electrode layer, thus acting to prevent the etching line width from unlimited spreading.
FIG. 9 illustrates a conventional electrolytic etching apparatus suitable for cases of etching the periphery of a rectangular substrate using the basic configuration of FIG. 8. FIG. 9 illustrates a configuration in which an etching electrode 904 mounted in a rectangle on a substrate upper portion is arranged having a narrow gap in the space between it and the substrate 905.
FIG. 10 illustrates an electrolytic etching apparatus, which further employs a rotating drum suitable for continuous processing in the apparatus shown in FIG. 9. In FIG. 10, reference numeral 1001 denotes an XY robot for substrate conveyance, reference numeral 1002 denotes a substrate absorption pad, reference numeral 1003 denotes a rotating drum and reference numeral 1009 denotes an air cylinder for etching electrode drive.
In this apparatus, the substrate is mounted on the rotating drum 1003 and is immersed in an electrolyte 1006 by rotating. After this, the substrate is moved by the air cylinder 1009 until the electrode 1004 strikes a strike pin (not shown), whereby a fixed space is secured from the substrate 1005. This space is usually designed to be extremely narrow, at about 0.1 mm to 1 mm, which prevents the electric field from leaking, whereby a fine etching line can be formed. After the electrode position has been decided, current is passed from the power source 1008 to carry out electrolytic etching.
As described above, to effectively extract the power of a solar battery, it is important that the etching lines be as fine as possible and be formed as far as possible on a cell edge so that the effective region can be as broad as possible.
However, in a conventional electrolytic etching method, it was impossible to form a fine line on a cell edge because of alignment problems with the electrode position accuracy and substrate position accuracy.
That is, if the alignment accuracy of the electrode position and the substrate position is, for example, 0.2 mm, unless either the etching line is widened by 0.2 mm more than the originally required width, or the electrode is positioned about 0.2 mm inside the originally required position, the etching pattern may not form at all on the substrate edge. In a region in which an etching pattern is not formed, because shunt portions in the cell remain, cell output worsens.
Trying to solve this problem by improving the alignment accuracy would require a large-scale mechanical apparatus, which makes implementation extremely difficult.
Because of the above-described points, forming a fine etching line on a cell edge was difficult, which was a limitation on effective extraction of power.